We propose to develop the forced random noise technique for measuring the frequency dependence of total respiratory impedance in infants and children, and to process this data to obtain total resistance, compliance and inertance and to fractionate total resistance into central and peripheral components. In a longitudinal study of normal children in a day care center, data will be collected routinely when the children are well and during all respiratory infection and convalescent periods. All respiratory infections will be fully characterized. Forced random noise parameters will be correlated with height, weight, sex and infection data in order to define normal lung growth and the effects of intercurrent respiratory infection on growth.